Variability of capacity demand is one of the main challenges to be addressed in wireless cellular deployments. In both macro-cell and small-cell deployments, capacity demand may be variable by location and by time of day (ToD), or to accommodate planned or unplanned events.
A macro-cell or macro-site is a large area cell in a mobile access network that provides radio coverage served by an outdoor high-power cell site comprising a base transceiver station (BTS), which may be referred to as a MACRO BTS, with high power antennas which are mounted on towers, masts, or other raised structures above the clutter, i.e. at a height that provides a line of sight (LOS) over the most of the surrounding buildings and terrain, with coverage having a range of 1 to 10 kilometers or more. Outdoor small-cells comprise lower power, more compact base-stations and antenna which are usually deployed below the clutter, e.g. mounted on existing street infrastructure, such as utility poles, strands, lamp posts, or on the sides of buildings, to provide localized enhanced coverage. For example, in dense urban areas small-cells may be used to fill coverage gaps, e.g. where tall buildings block coverage by a macro-cell, or to provide enhanced service to meet capacity demand for high traffic environments, or hotspots, at street level. More generally, small-cell deployments may include what are referred to as micro-cells, pico-cells and femto-cells. Micro-cells provide coverage for smaller areas than macro-cells, e.g. providing coverage with a range of hundreds of meters, extending several city blocks, over plazas and event spaces, transit hubs, etc., and with capacity for serving hundreds of users. Pico-cells and femto-cells cover smaller outdoor and indoor areas and serve a more limited number of users.
In the MACRO BTS world, where installations are mounted on a tower or on top of a building, the issue of capacity demand tends to be addressed by using a distributed architecture and/or an Adaptive Antenna System (AAS).
In a conventional distributed radio access network (RAN) architecture, a radio transceiver is mounted on an antenna tower or mast, close to the antenna, and connected to a ground level base transceiver station (BTS) by copper or coax cable. That is, the high-power radio and antennas are mounted on the tower, separated from the baseband processor unit, which is located the foot of the tower, in a cabinet that includes the power supply and an active cooling system. The radio unit may be referred to as RRU (Remote Radio Unit) or RRH (Remote Radio Head). The baseband processor unit may be referred to as a BBU (baseband unit). In cloud RAN architectures (C-RAN), a transceiver and antenna are mounted on a tower or mast and connected by a fiber link (fronthaul link), e.g. using CPRI (Common Public Radio Interface), to an optical BBU located at a central office, or other centralized BBU location some distance away, which also serves other transceivers and antennas, and provides a centralized base-station control system (BCS). Each BBU serves multiple RRHs, so that capacity can be routed to RRHs as required. This CPRI architecture allows for a flexible distribution of baseband resources to radio resources.
AAS solutions use multiple antenna elements for transmitting and/or receiving, which can be configured to cover multiple use-case scenarios using technologies, such as, multi-beam antennas and advanced full-dimension (FD) digital beamforming.
Carrier Aggregation (CA) may be used to address capacity demand. Carrier aggregation is a scheme for transmitting data to a terminal or User Equipment (UE) multiple different unit carriers, or component carriers (CC), to increase data rates. As an example, LTE-A release 8/9 provides for carrier aggregation of up to 5 CC, each having a bandwidth of 20 MHz and providing up to 130 Mbps per CC, to extend the bandwidth to 100 MHz. Carrier aggregation may be intra-band contiguous aggregation, intra-band non-contiguous aggregation and inter-band non-contiguous. Thus, using a combination of CPRI architecture, AAS using multiple antenna elements, and carrier aggregation, network capacity for macro-cell deployments can be steered to a specific RRH and antenna elements to cover an area where capacity is required.
The following references provide some examples of apparatus and methods for carrier aggregation for uplink and downlink: U.S. Pat. No. 9,036,583 entitled “Transmission method and apparatus for carrier aggregation and uplink MIMO”, issued 19 May 2015 (Lim et al.); US20120294299A1 entitled “Non-Adjacent Carrier Aggregation Architecture” published 22 Nov. 2012 (Fernando); US20130051284A1 entitled “Carrier Aggregation Radio System” (Khlat) published 28 Feb. 2013; US20140227982A1 entitled “Front End Circuitry for Carrier Aggregation Configurations” (Granger-Jones et al.) published 14 Aug. 2014.
The system architectures and solutions used for MACRO BTS may not be applicable to small-cell deployments for several reasons, e.g.: high costs, large form factor, high power consumption, active cooling, and in particular, deployment use-case.
As mentioned above, antennas for macro-cells are mounted high above the clutter, on masts, towers, or on tall buildings and use, for example, large scale tri-sector antenna to provide high power coverage over an extended range of the surrounding area, for various network morphologies, e.g. urban areas or rural terrain. Moreover, using a common BTS platform connected by CPRI to multiple RRHs, capacity can be routed at baseband level towards different RRHs. In this way, processing power required for CA can be used towards one RRH or another.
Small-cells differ in that the baseband processor and radio components, i.e. the RF transceiver and RF front end, are fully integrated into a small form factor unit, with an integrated antenna or attached antenna. In an LTE Radio Access Network, small-cell units may be referred to as eNodeB or eNB. In high density urban areas, outdoor small-cells are usually deployed at street level, e.g. mounted on a pole or lamp-post at about 18 feet above the ground. In most cases, surrounding buildings are at the same height and there may be tall buildings that are significantly higher than the installation point. For this reason, small-cell antenna patterns are highly influenced by the clutter and usage of sophisticated beamforming technologies is not very efficient. Thus, small-cells typically use omni-directional or quasi-omni-directional antenna architectures as a compromise for different street level use-cases, e.g. as a trade-off for the canyon effect and location dependent propagation effects.
There is a need for alternative solutions which improve or optimize current small-cell architectures. For example, there is a need for a cost effective, low form factor, small-cell architecture that provides for carrier aggregation, and enables one or more of: improved or optimized cell coverage, improved user experience and fit to any network morphology.